This invention relates to non-impact printing, photocopying and like reprographic processes and paper for use in such method.
The term "non-impact printing" is used herein to encompass various methods of forming an image on paper by means other than transferring an ink image mechanically from a carrier which strikes the paper forceably to imprint the ink on the paper. In particularly, but not exclusively, examples of non-impact printing processes include on the one hand thermal and electrostatic or electrosensitive imaging on appropriately sensitised papers, and on the other hand ink jet or toner deposition normally on plain paper, the most common toner based technology involving electrostatic transfer of toner to the paper followed by fusion of the deposited toner to form a permanent image. Similar techniques are employed in photocopying and other like reprographic processes, and the term "non-impact printing" is also intended to include such other processes.
In many cases, it is also required to print by these techniques onto paper having a sensitive layer (for example sensitive to pressure, heat, light, electric charge etc.) for other purposes. In particular there is currently a requirement, for example in the production of multi-part sets, to print by various non-impact processes onto paper which carries a pressure sensitive layer in the form of an appropriate coating to enable the paper subsequently to be used for forming copies, without the use of carbon paper or the like, when used for conventional typing onto the pre-printed paper. These so-called "carbon-less" copy papers generally rely on two coatings formed respectively on the contiguous faces of superimposed sheets of paper, namely a coating containing a colour-forming substance, usually contained in micro-capsules, on the back of the uppermost sheet (usually known as a CB coating) and a coating of a receptor layer on the front of the lowermost sheet (usually known as a CF coating). Colour-forming chemicals are typically dissolved in an oily solvent and encapsulated by well known techniques, and when such capsules are ruptured by mechanical pressure, as by impact of a type bar of a typewriter, the chemicals are released and react to form a visible mark on the CF coating of the adjacent sheet.
Conventially, paper for use in such copying systems is of three types, distinguished by their coatings, namely CB sheets having a CB coating on the underside to form the top sheet of the set, CF sheets having a CF coating on the upper side to form the bottom sheet of a set, and optionally CFB sheets having a CF coating on the upper side and a CB coating on the underside to form one or more intermediate sheets of a set where required. Such coatings are normally applied by a continuous process to cover the entire area of the appropriate face of the sheet.
In recent years, there has been a demand for printing onto such coated paper as the recipient material in tone-based printing systems in which an image is transferred to the recipient material electrostatically, a process sometimes known as "electronic or laser printing". However, difficulties have been experienced in such electrostatic printing systems when the recipient material comprises such coated papers. In particular, it appears that the handling of CB material, either before or during the printing process, may rupture some of the capsules containing the colour-forming chemicals, and this is particularly liable to lead to "poisoning" of the electrostatic transfer drum and also contamination of paper-feed rollers so that the printing apparatus fails to perform satisfactorily after a short period of operation using such CB (or CFB) sheets. No satisfactory solution to these problems has yet emerged.
Likewise other special paper coatings may interfere with the correct operation of various forms of non-impact printing apparatus or the printing process.
The present invention seeks to overcome such problems in a surprisingly simple and effective manner.